This study was designed to evaluate the effect of feeding grade level of dietary protein (36, 32, 28 and 24%) on the growth and immune responses of pacific white shrimp for 51-100 days of culture period. In an outdoor pond trial, L. vannamei postlarvae-15 stage (0.0023 g) were stocked into a 50 m2 polyethylene-lined pond at a density of 100 shrimp/m2. This experiment was divided to four treatments with four replicates per treatment. In treatment-1, shrimp were fed 36%CP throughout the culture period. In treatment-2, -3 and -4, shrimps were fed 36%CP for the first 50 days, after that the shrimp were switched to 32, 28 and 24%CP feed, respectively, until day-100. The result showed the weight gain (WG), average daily gain (ADG), survival rate (SR) and specific growth rate (SGR) of shrimps were not affected by treatment (P>0.05). However, in terms of productivity, treatment-1 (74.94 kg.pond-1) and treatment-4 (74.70 kg.pond-1) were significantly lower as compared with treatment-2 (94.1 kg.pond-1). Feed utilization, protein efficiency ratio (PER) and protein retention (PR) of treatment-1 was significantly lower than treatment-3 and -4. For the immune responses of shrimp, total hemocyte count (THC), phenoloxidase activity (PO) and the production of superoxide anion (SO), as happens during the respiratory burst in many immuno-competent cells, on day 60 and 100 of culture were not significant different among the treatments. From these results, it is concluded that the optimal dietary protein level for L. vannamei, after day 50 or body weight over 7.5 g can be lowered from 36% to 24-32%, because the growth performance and the immune responses of shrimp were not different from feeding only 36%CP diet throughout the culture period.
